1. Field of the Invention
The present invention relates to thin film materials, suitable for ferroelectric tunable devices and/or decoupling thin film capacitors based on such materials, and methods of manufacturing thin film devices, and more particularly, to paraelectric perovskite oxynitride nanocomposite materials and methods of making such materials for use in forming varactor devices with improved voltage tunabilities and high capacitance density thin film decoupling capacitors.
2. Description of the Related Art
In order to achieve maximum tunability of a ferroelectric varactor, a maximum voltage must be applied to induce a change in the dielectric constant needed to produce the maximum possible shift in capacitance. FIG. 1 illustrates an example of a typical tunability achieved with a ferroelectric varactor using a thin-film ferroelectric material, such as (Ba,Sr)TiO3. FIG. 1 corresponds to FIG. 7 of U.S. 2009/0069274.
U.S. 2009/0069274 discloses a tunability of 2.8:1 under an electric field of 450 kV/cm to 500 kV/cm or a 70% reduction of the original capacitance under a 450 kV/cm bias field as shown in FIG. 1. Additional examples reported typical tunabilities for 100 nm to 200 nm thick (Ba,Sr)TiO3 (BST) films used in parallel plate Pt/BST/Pt MIM type variable capacitors (varactors) in the range of 4:1 at 11 Volts (see, for example, FIG. 5 of U.S. 2007/0069274) to 6.3:1 at 10 Volts as shown in FIG. 2, which corresponds to FIG. 5 of T. Bernacki, I. Koutsaroff, and C. Divita, “Barium Strontium Titanate Thin-Film Multi-Layer Capacitors”, Passive Component Industry Magazine, September/October 2004, pp. 11-13.
Oxynitrides perovskites can often be described as derivatives of oxides, formed by simultaneous substitutions (charge equivalency (balance) rule) of cation and anion components. The higher anionic charge resulting from the N3−/O2− substitution can be compensated according to two different principles. In the first case, a cross-substitution is applied with trivalent RE3+ (rare earth) elements as suitable substitutes, for instance, for divalent alkaline-earth cations. For example, the oxynitride “charge balance equivalent” to BaTiO3 will be LaTiO2N1, or NdTiO2N1. Another example for charge compensation in AB(ON)3 oxynitrides perovskites, is simultaneous substitution of the Ti4+ with Me5+ and partial substitution of O2− sites with N3− so as to convert the perovskite oxide BaTiO3 into the oxynitride perovskites, such as BaTaO2N or BaNbO2N, in addition to LaTaON2.
The incorporation of N3−/N2− into oxygen anion sites of the perovskite oxides results in pronounced structural effects, such as an elongated Ti(Zr)—N bond length and the reduced electronegativity of the nitride ion N3−, with respect to the oxide ion O2−, which will tend to increase the covalence of the cation-anion bonds. The increased covalence of the bonding can in turn increase the likelihood of cation displacements through a second order Jahn-Teller-like distortion of the d0 cation and could influence the ferroelectric properties of the oxynitride perovskites by suppressing the formation of a ferroelectric phase and enhancing the paraelectric properties into a superparaelectric state. Even the oxynitride formation could be associated with a structural change from cubic symmetry (Pm3m) to non-cubic (e.g., tetragonal) or quasi-cubic with increased in the tetragonal distortion (c/a ratio). On the other hand, the mixed occupancy of the anion site in oxynitrides AB(O1−xNx)3, provides a condition similar to that found in relaxors, as the polarizing octahedral cations (Ti4+) will experience random chemical environments in the absence of complete O/N sites ordering. Anion control has previously been utilized to tune the properties of ferromagnetic and paramagnetic perovskite or double perovskite materials.
Most recently both N2 and NH3 containing plasmas have been used for the nitridation of cubic perovskite single crystals, bulk ceramic, and thin film samples, such as SrTiO3, and for PLD and RF-sputtered depositions of BaTaO2N1, as well as growth of LaTiO2N1 epitaxial thin films on SrTiO3 or MgO substrates from oxynitride targets. However, there have been no reports of deposition and characterization of oxynitride polycrystalline ABO2N1 or ABO3-γNγ thin films grown on Pt electrodes on common large size commercially available substrates nor any C-V or I-V characteristics of any ferroelectric oxynitride perovskite, except for the dielectric constants of LaTiO2N1 and BaTaO2N1 thin films at zero dc bias. In addition, even epitaxially grown BaTaO2N films at 760° C. from a oxynitride target on a SrTiO3:Nb substrate with a SrRuO3 buffer by PLD method with gas ratio of N2/O2 of 20:1 had a dielectric constant of only 220, which is about 20 less than that of BaTaO2N bulk samples. Temperature coefficient of capacitance (TCC) of BaTaO2N films from 10K to 300K is in the range of −50 ppm/K to 100 ppm/K.
For the case of RF sputter-deposited LaTiO2N1, the dielectric constant had been reported to be from 400 to 1100 without any bulk ceramic data shown for comparison and without any voltage tunability or TCC data.
It had been previously observed that that the presence of N2 in the plasma reduces the surface defects on the electrodes as well as reducing the leakage current with almost no noticeable enhancement of the dielectric constant in SrTiO3 films for low deposition temperatures (200° C.). The observed lower leakage (higher insulation resistance) in N-doped SrTiO3 films had been attributed to nitrogen substitution of the oxygen vacancies generated by the high deposition rate of the SrTiO3, and N compensation of the donor sites created by the oxygen vacancies, without any further evidence or actual mechanism causing the lower leakage.
All of the commonly known deposition methods of BST films, and particular solutions for achieving high voltage tunability and/or high capacitance density required for achieving better performance variable capacitors and/or high density decoupling thin film capacitors, typically require using very high deposition temperatures of about 800° C. or higher, very high post-deposition annealing temperatures between 800° C. and 900° C., and thicker BST dielectric layers, typically between 200 nm and 600 nm, all of which make it very difficult to simultaneously achieve large volume manufacturing reproducible quality paraelectric thin films with reasonably high tunability ratios, i.e., tunability ratios of at least 4-6:1, under applied DC biases below 6-8 Volts, low dielectric loss, i.e., of less than 1% at 1 KHz or 1 GHz, which is typically only possible at lower deposition temperatures of about 600° C. to about 650° C. The deposition of oxynitride perovskite thin films requires using epitaxially matching substrates which are not available in large manufacturing sizes and typically obtained oxynitride perovskite materials are not stable above 600° C. if annealed in oxygen atmospheres.